1. Field of the Invention
The present invention relates to an optical tomographic imaging apparatus for obtaining a tomographic image of a measuring object by OCT (optical coherence tomography) measurement.
2. Description of the Related Art
An optical tomographic image obtaining system using OCT measurement is sometimes used to obtain an optical tomographic image of a living tissue. It is applied to diagnosis of various regions ranging from fundus or anterior chamber of an eye, or skin to the observation of artery wall using a fiber probe or observation of a digestive organ using a fiber probe inserted through the forceps channel of an endoscope. In the optical tomographic image obtaining system, a low coherence light beam outputted from the light source is split into measuring and reference beams, and the measuring beam is irradiated onto a measuring object, then the reflected beam reflected from the measuring object or backscattered light when the measuring beam is irradiated thereon is combined with the reference beam, and an optical tomographic image is obtained based on the intensity of the interference beam between the reflected beam and the reference beam. Hereinafter, reflected beam reflected from a measuring object and backscattered light are collectively referred to as the “reflected beam”.
In the mean time, FD (Fourier Domain)-OCT measurement measures the intensity of interference beam with respect to each spectral component of the light without changing the optical path lengths of the reference beam and signal beam, and performs frequency analysis, typically a Fourier transform, on the obtained spectral interference signals using a computer to obtain a reflected beam intensity distribution corresponding to depth positions. The FD-OCT does not require the mechanical scanning used in TD-OCT, so that it has been drawing wide attention as a method that allows high speed measurement.
Typical systems that use FD-OCT measurement are SD-OCT (Spectral Domain OCT) system and SS-OCT (Swept Source OCT) system. The SD-OCT system uses a broadband and low coherence light beam, such as SLD (Super Luminescence Diode), ASE (Amplified Spontaneous Emission), or white light beam, as the light source, and forms an optical tomographic image in the following manner. The broadband and low coherence light beam is split into measuring and reference beams using Michelson interferometer or the like and the measuring beam is irradiated onto a measuring object, then a reflected beam reflected from the measuring object when the measuring beam is irradiated thereon is caused to interfere with the reference beam and the interference beam is broken down into frequency components using a spectrometer, thereafter the intensity of the interference beam with respect to each frequency component is measured using a detector array including elements, such as photodiodes, disposed in an array and an optical tomographic image is formed by performing Fourier transform on the obtained spectral interference signals using a computer.
In the mean time, the SS-OCT system uses a laser that temporally sweeps the optical frequency, in which the reflected beam is caused to interfere with the reference beam at each wavelength, then the temporal waveform of the signal corresponding to the temporal change in the optical frequency is measured and an optical tomographic image is formed by performing Fourier transform on the obtained spectral interference signals using a computer.
For the OCT system, in order to obtain high resolution and high quality image, it is necessary to broaden the wavelength of the light source and to increase the number of corresponding data points. In the SD-OCT system, however, the interference beam is generally detected with respect to each wavelength using a detector array including elements, such as photodiodes, disposed in an array, so that the number of data points is limited by the number of elements of the detector array. At present, it is not desirable to increase the number of elements of the detector array for increasing the number of data points, since such increase would result in cost increase, decreased manufacturability, reduced measuring rate, and the like. On the other hand, in the SS-OCT system, in order to increase the number of data points, for example, it is just necessary to increase the sampling frequency of the circuit that converts an optical current signal from the detector to a digital value if the frequency sweep period of the light source is assumed to be constant, so that it may be realized easily at low cost with a high measuring rate.
In various OCT measurements described above, it is known that the measuring beam with a broad spectral width is used in order to improve spatial resolution as described, for example, in Japanese Unexamined Patent Publication No. 2002-214125. This patent publication discloses a light source including a plurality of light sources, each emitting a light beam having a different spectral range, and an optical coupler for combining the light beams emitted from the respective light sources to emit a single-wave light beam as a light source capable of emitting a light beam having a broad spectral width.
For the SD-OCT measurement, a method for forming a continuous spectrum by combining light beams from a plurality of gain media, each having a overlapping wavelength range with each other, is disclosed in Japanese Unexamined Patent Publication No. 2001-264246. As for the method of forming a continuous spectrum through wavelength combination for SS-OCT, a structure including a plurality of wavelength scanning light sources, each having a gain medium and a wavelength selection element is disclosed in Japanese Unexamined Patent Publication No. 2006-047264. Further, U.S. Pat. No. 6,665,320 discloses a structure that controls light beams from a plurality of gain media using a single wavelength selection element.
Where light beams from a plurality of light sources are combined and used in order to obtain high spatial resolution, a conventional SS-OCT system poses a problem that, when light beams having different wavelengths are outputted from a plurality of light sources and irradiated onto a measuring object at the same time, the interference information provided by the plurality of light beams is mixed up and unable to be detected since the detector of the system includes only a single element.
For this reason, in the systems described in Japanese Unexamined Patent Publication No. 2006-047264, and U.S. Pat. No. 6,665,320, a configuration is adopted in which only a single wavelength is inputted to the detector at a time by controlling the light source or using a switching element. Such method, however, poses a problem that the measuring rate is reduced since it takes time to irradiate all of the wavelengths of the measuring beam, though it may provide broadband light as the measuring beam.
The present invention has been developed in view of the circumstances described above, and it is an object of the present invention to provide an optical tomographic imaging apparatus capable of obtaining a high resolution tomographic image rapidly.